1. Field of the Invention
The present invention relates to the structure of an infrared sensor employing thermopile or other elements.
2. Description of the Prior Art
There are known various structures of non-contact type infrared sensors which utilize thermopile elements, thermistors or current collector elements. An infrared sensor utilizing a diaphragm-like thermopile element, which will be called "thermopile type infrared sensor", is disclosed, for example, in U.S. Pat. No. 4,111,717. The basic structure of a thermopile element used in the thermopile type infrared sensor is shown in FIGS. 1 and 2 of the accompanying drawings. FIG. 1 is a cross-sectional view of the thermopile element of the thermopile type infrared sensor while FIG. 2 is a plan view of the primary part of the thermopile element.
Referring to FIG. 1, the thermopile element comprises a silicon chip 10, one side of which is treated and coated with an insulation film 11. The central portion of the silicon chip 10 is then etched to form a pit 12. The remaining part of the insulation film 11 within the pit 12 will defines a diaphragm 13. The insulation film 11 may be of a single layer of silicon oxide (SiO.sub.2) or three-layer film consisting of a silicon nitride (SiN.sub.2), a silicon oxide (SiO.sub.2) and a silicon nitride (SiN.sub.2).
A number of thermocouples 14 are disposed on the insulation film 11 and electrically connected in series with each other. Each of the thermocouples 14 includes a hot junction 14a placed on the diaphragm 13 and a cold junction 14b positioned on a heat sink 11a which is defined by the remaining part of the silicon chip 10 placed around the outer periphery of the diaphragm 13 and having a larger heat capacity.
The central portion of the diaphragm 13 carries a black body 15 for absorbing infrared rays. Generally, the black body 15 may be formed by ultrafine gold particles deposited over the diaphragm 13 and which look black since these gold particles absorb visible rays and infrared rays without reflection. Since the gold is an electric conductor, the black body 15 must be reliably separated away from the hot junctions 14a of the thermocouples 14 without any short-circuiting.
As the black body 15 of such an infrared sensor absorbs infrared rays from an object to be measured, temperature rises in the diaphragm 13 at the hot junctions 14a of the thermocouples 14. Thus, a difference in temperature between the hot and cold junctions in each of the thermocouples 14 will create a voltage output. Since the amount of infrared ray is interrelated with the temperature of the object, the thermopile element will serve as a non-contact type thermometer. On conversion of temperature, however, the temperature at each of the cold junctions of the thermocouples 14 must be accurately measured at all times since this temperature is used as a reference level.
One of simple methods for measuring the temperature at the cold thermocouple junction is that a thermistor is mounted on the outside of a package on which the thermocouples 14 are mounted. In such a case, however, the thermistor is not in direct contact with the cold junction of each of the thermocouples. There is thus created a temperature gradient which will prevent the temperature at the cold junction from being accurately monitored.
In order to overcome such a problem, it has been considered that the thermistor is housed within the package in proximity to the cold junction. The thermistor is miniaturized or formed into a film and mounted on the heat sink 11a on which the cold junction is formed. However, the heat sink 11a has only a very small space which is utilized to mount the thermistor. Therefore, the size of the thermistor should be reduced into about 1/10 times the size of the pile element 1. Since the pile element 1 itself is at most of two millimeter square, the reduction of size in the thermistor is very difficult, involved with less reliability.
On the other hand, such a thermopile type infrared sensor requires a diaphragm 13 of a material having a heat conductivity as small as possible to take a differential temperature between the hot and cold junctions 14a and 14b of a thermocouple 14 more easily. When the diaphragm 13 having less heat conductivity is used, heat emitted from the object to be measured and absorbed by the black body 15 will not transmit to the hot junction 14a very well. This results in reduction of sensitivity.
As shown in detail in FIG. 2, each of the thermocouples 14 comprises a pair of first and second conductors 141 and 142 which are made of different metallic materials and connected in series with each other. The hot junction 14a of the thermocouple 14 is disposed on the diaphragm 13 while the cold junction 14b thereof is located outside of the diaphragm 13, that is, on the heat sink 11a. The central portion of the diaphragm 13 carries an electrically conductive black body 15 which is separated apart from the hot junction 14a of each of the thermocouples 14 normally several tens microns for providing an electrical insulation. Energy of the infrared rays absorbed by the black body 15 will be consumed to increase the temperature of the diaphragm 13 itself as it moves to the hot junction 14a of the thermocouple 14 via the diaphragm 13. As a result, the temperature of the hot junction 14a cannot be increased to a desired level which provides a desired output voltage. In order to overcome such a problem, it has been proposed that the number of thermocouples is increased to increase the output voltage. However, such a proposal leads to increase of cumbersome steps and still provides less reliability. It has been also made such a proposal that the black body is made of an insulation material. However, such a technique does not reach a level that its characteristic is acceptable.